Is it a grand prix GXP? Our cars dont have the theftlock that disables when we take out our battery since the radio and sattelite reciever match the stored vin number to that of the BCM Body Computer to recognize if they couold be stolen or not. So you can unhook the battery as many times as you like.
Even though the manual states theres a backup battery for the onstar...if it is a grand prix I really doubt it has one. They make the manuals very generic and ive seen a few errors throughout them, I know the impala's have them and some other vehicles. If it does it looks like a little black box with two wires almost next to the onstar module( which on the grand prix is a silver module just behind the climnate control) you can see it by looking up under neath the center of the dash from the floor.
I also just looked at the schematic for 08 gxp onstar and confirmed there is no backup battery.
On the soundgate device I never had experience with them but possibly someone else could comment on them,
